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Monday, June 25, 2012

Asteroids and bear traps

A giant pink seastar, Pisaster brevispinus, had washed up with the incoming tide, and was lying in a half inch of water, probably dying. These stars, unlike the smaller purple starfish, don't do very well out of the water.

It measured 19 inches, tip to tip. They may grow to 2 feet across.

Flipped over. It made no attempt to turn back right side up.

Since it was so lethargic, we were able to take our time and get a few close shots.

Section of one of the arms, showing the ambulacral groove filled with tube feet, and the bony spines along the sides.

The tube feet are connected to the internal water system; the starfish pumps water into the feet to extend them. They excrete an adhesive to grip the surface, and an anti-adhesive to release it. The glue is strong; add in the suction produced by the hydraulics, and the starfish can exert a pull of up to 9 pounds per square inch. It is almost impossible to remove a clamshell from a determined starfish without ripping off the tube feet holding it.

The mouth is in the centre of the body, opening on the underside. The starfish has no teeth, no radula to scrape with, no jawbones. Instead, it extends the stomach outwards, trapping and partially digesting its prey before it transfers it to the inner stomach. (It has two.)

Partially everted stomach.

When feeding on bivalves (e.g. muscles, oysters) these seastars will prize the valves apart (using their tube-feet suckers to gain a hold) until they open by as little as 0.1 mm and then they will evert their stomach through this gap and digest their prey! (From Cronodon.com)

I should really have had a lens with me. Until we were looking at the photos at home, we had not seen the "bear traps", as some describe them.

See those little two-pronged pincers? Those are the "bear traps".

Pedicellariae (the scientific name for these) in starfish come in two sizes. What are visible here are the large ones. Around the base of each bony spine is a clump of miniature ones, as well; they are the pale orange dots that fill the gaps. (If you click on the photo to see it full size, then enlarge it, the shapes are just visible.)

The pedicellariae essentially "go off" when food hits them. So, some small shrimp, krill or other tasty bit of organic, edible goodness?? BAM! Snagged by the spine/pedicellariae!! Prey are held fast by the pedicellariae similar to velcro (to use Emson & Young's terminology). (From The Echinoderm)

Captured critters, such as amphipods, barnacle larvae, and even some small fish, can be passed on from one group of traps to the next, transferring them eventually to the mouth.

Other slow-moving animals often collect a living cargo of seaweeds and other algae, barnacles and limpets and tubeworms, amphi- and isopods, as well as assorted parasites. Not the starfish; they're always clean. The pedicellariae make sure of that.